What is Chemical Equilibrium?

Chemical equilibrium is a state in a chemical reaction where the concentrations of the reactants and products remain constant, with no net change in the amounts of the substances present.

Chemical equilibrium refers to a state in which the concentrations of reactants and products remain constant over time, and the system displays no further change in properties.

Table of Contents

Types of Chemical Equilibrium

Factors Affecting Chemical Equilibrium

Examples of Chemical Equilibrium

What is the Importance of Chemical Equilibrium?

Problems on Chemical Equilibrium

When the rate of the forward reaction is equal to the rate of the reverse reaction, the system is said to be in a state of chemical equilibrium. No further change in the concentrations of the reactants and the products occurs due to the equal rates of the forward and reverse reactions, and the system is said to be in a state of dynamic equilibrium.

State of Chemical Equilibrium

A graph with the concentration on the y-axis and time on the x-axis can be plotted. Once the concentration of both the reactants and the products stops showing change, chemical equilibrium is achieved.

What is Dynamic Equilibrium in Chemical Equilibrium?

At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction. This means that the number of reactant molecules converting into products and product molecules into reactants are the same. However, the same equilibrium can be maintained in different conditions, with continuous interchanging of molecules, thus making chemical equilibrium dynamic.

Introduction to Chemical Equilibrium

Chemical Equilibrium

Types of Chemical Equilibrium

There are two types of chemical equilibrium:

1. Dynamic equilibrium
2. Static equilibrium

Bold Homogeneous Equilibrium

Heterogeneous Equilibrium

Homogeneous Chemical Equilibrium

In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous equilibrium can be further divided into two types: Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,

2H₂(g) + I₂(g) ⇌ 2HI(g)

2N₂(g) + O₂(g) ⇌ 2NO₂(g)

Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules are known as unequal-molecule reactions. For example,

2SO2 (g) + O2 (g) → 2SO3 (g)

COCl2 (g) <=> CO (g) + Cl2 (g)

Heterogeneous Chemical Equilibrium

Examples of heterogeneous equilibrium include:

• Dissolution of ammonia in water
• Formation of silver chloride
• Formation of Iron (III) hydroxide

2CO (g) ⇌ CO2 (g) + C (s)

CaCO3 (s) <=> CaO (s) + CO2 (g)

Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.

⇒ Check: Ionic Equilibrium

Factors Affecting Chemical Equilibrium

Factors Affecting Chemical Equilibrium

According to Le-Chatelier’s principle, if there is any alteration in the components influencing the equilibrium conditions, the system will attempt to counter or minimize the influence of the overall transformation. This rule is applicable to both chemical and physical equilibrium.

Some of the important factors affecting chemical equilibrium are temperature, pressure, and concentration of the system.

Change in Concentration: denotes italics

This is an example of italicized text.

The concentration of the reactants or products is reduced by the reaction, which consumes the substance that was added.

The reaction proceeds in a direction that replenishes the substance which has been removed, thus relieving the concentration of reactants or products.

When the concentration of the reactant or product is altered, the composition of the mixture in chemical equilibrium is affected.

Impact of Change in Concentration

Change in Pressure:

If the volume changes, then the pressure will also change. This can have an effect on gaseous reactions as the number of gaseous reactants and products will be different. However, according to Le Chatelier’s principle, changes in pressure in both liquids and solids can be ignored as the volume is not affected by pressure.

Impact of Alterations in Volume of Equilibrium Mixture

Increase in Temperature:

Le Chatelier’s Principle states that the effect of temperature on chemical equilibrium depends upon the sign of ΔH of the reaction.

As temperature increases, the equilibrium constant of an exothermic reaction decreases.

In an endothermic reaction, an increase in temperature leads to an increase in the equilibrium constant.

As per Le Chatelier’s principle, the rate of reaction as well as the equilibrium constant are both affected by the change in temperature. In the case of exothermic reactions, the equilibrium shifts towards the reactant side when the temperature increases, whereas for endothermic reactions, the equilibrium shifts towards the product side with an increase in temperature.

Impact of a Catalyst:

A catalyst does not affect the chemical equilibrium; it only speeds up a reaction. In fact, catalyst equally speeds up the forward as well as the reverse reaction. As a result, the reaction reaches its equilibrium faster.

The same amount of reactants and products will be present at equilibrium in a catalysed or a non-catalysed reaction; however, the presence of a catalyst will reduce the energy required to reach equilibrium. The catalyst allows the reaction to proceed through a lower-energy transition state of reactants to products.

Impact of Adding an Inert Gas

When an inert gas like argon is added to a constant volume, it does not take part in the reaction, so the equilibrium remains undisturbed. However, if the gas added is a reactant or product involved in the reaction, then the reaction quotient will change.

Effects of Adding an Inert Gas

Examples of Chemical Equilibrium

1. Haber Process
2. Ammonia Synthesis
3. Nitric Acid Production
4. Carbonic Acid-Bicarbonate Buffer System

Reactants and Products are two states in a chemical reaction; the forward reaction converts reactants into products, while the backward reaction converts products back into reactants. These two states are different in composition.

After some time of the start of the reaction, the rate of the forward and the backward reactions may become equal. At this point, the number of reactants converted will be reformed by the reverse reaction such that the concentration of reactants and products remain constant. Therefore, the reactants and products are in chemical equilibrium.

2NO2 ⇌ N2O4

PCl3 + PCl2 ⇌ PCl5

2N_2 + 3H_2 ⇌ 4NH_3

The Significance of Chemical Equilibrium

It is useful in many industrial processes such as

Preparation of Ammonia by Haber’s Process: In this process, nitrogen combines with hydrogen to form ammonia. The yield of ammonia is higher at lower temperatures, higher pressure, and in the presence of iron as a catalyst.

Preparation of Sulphuric Acid by Contact Process: In this process, the fundamental reaction is the oxidation of Sulphur Dioxide (SO2) into Sulphur Trioxide (SO3). This involves a chemical equilibrium.

Equilibrium Solved Questions

Top 7 Questions on Ionic Equilibrium

Problems on Chemical Equilibrium

1. The equilibrium constant KP for the reaction N2 (g) + 3H2 (g) ⇌ 2NH3 (g) at 500oC, given that the heat of the reaction at this temperature range is -25.14 kcal, is estimated to be 1.06 × 10-4atm-2.

Given:

This is a heading

Solution:

This is a heading

The equation relating equilibrium constants at different temperatures and the heat of the reaction is:

log KP2 = -25140/2.303 × 2 \[773 - 673 / 773 × 673\] + log 1.64 × 10^{-4}

log_{KP2} = -4.835

KP₂ = 1.462 × 10^-5 atm^-2

Given the equation, N2 (g) + 3H2 (aq) ⇌ 2NH3 (g), Find Q and determine which direction the reaction will shift in order to reach the state of chemical equilibrium:

To find Q, we need to calculate the concentrations of each species at equilibrium. Once we have these values, we can plug them into the equilibrium expression to calculate Q. Once we have Q, we can compare it to K, the equilibrium constant, to determine which direction the reaction will shift in order to reach the state of chemical equilibrium.

Given:

This is a heading

Solution:

This is a Heading

Given, [N2] = 0.04M, [H2] = 0.09M, and K = 0.040

Since only nitrogen and hydrogen concentrations are given, it can be assumed that they are the reactants and ammonia is the product. Since ammonia concentration is not given, it can be assumed to be zero.

As q is the ratio of the relative concentration of products to reactants, here q = 0.

K > Q, so Nitrogen and Hydrogen will combine to form Ammonia.

Also Read:

Equilibrium By Changing The Concentration

Shift In Equilibrium By Increasing/Decreasing The Concentration Of Ferric Ions And Thiocyanate Ions

Chemical Equilibrium: All Concepts and JEE Questions

Top 15 Most Important and Expected JEE Questions on Equilibrium

Frequently Asked Questions (FAQs)

How does temperature affect the equilibrium constant of an exothermic reaction?

The effect of a catalyst on a chemical equilibrium is to shift the equilibrium in the direction that favors the reaction with the lower activation energy, thus increasing the rate of the reaction and allowing it to reach equilibrium faster.

A catalyst has no effect on the chemical equilibrium.

What is the effect of addition of an inert gas during chemical equilibrium?

There is no effect on chemical equilibrium on addition of inert gas. What is meant by forward reaction?

A backward reaction is a reaction in which the products are converted to reactants.

A reaction in which the reactants are converted back to products is called a backward reaction.